The present invention relates to a mail-article processing apparatus, and more particularly to a mail-article tracing apparatus for use in an automatic mail-handling system.
One example of an automatic mail-handling system requiring a mail-article tracing apparatus is a postal code printing apparatus, in which an address described on a mail-article is picked up by an operator or an optical character reader (OCR), and the postal code corresponding to this address is printed on the same mail-article.
In a prior art mail-article tracing method, plural detectors such as photo-electric detectors are provided along a mail transport path such that there is a predetermined distance between adjacent mail detectors, and plural registers are also provided corresponding to the plural mail detectors, respectively. In this method, a letter is traced by shifting a signal showing a mail-article in the series of registers in response to the detection at the series of the mail detectors. In this case, since the register can store only one signal representing one mail-article, a plurality of mail-articles should not be presented in the transport path corresponding to a section between two adjacent mail detectors. This means that the length of the section should be shorter than the minimum transfer interval of the mail-articles. Therefore, a great number of mail detectors must be provided in a mail-handling system which requires a long transport path. The tracing method just described is referred to as the light bar method hereinafter.
There is another prior art mail-article tracing method reducing the number of the mail detectors in the light bar method, in which a series of several mail detectors are omitted, and a timer circuit is provided to produce a psuedo mail detecting signal. In the timer circuit, the psuedo mail detecting signals are produced in substitution for the mail detection signals delivered from the omitted mail detectors by counting the clock pulses generated in response to the transport speed to a predetermined amount which corresponds to the distance between two adjacent mail detectors in the light bar method. This tracing method is defined as a clock method hereinafter. In the clock method, since the timing of the psuedo mail detecting signal outputted from the timer circuit can be considered equivalent to that of the mail detection signal delivered from the omitted mail detector in the light bar method, the transport speed, for example, the transport belt speed must be kept equal to the actual mail transport speed. However, the actual mail transport speed is in practice because of the slippage between the mail-article and a transport path, and this variation in the actual mail transport speed results in a timing error between the psuedo mail detecting signal and the time when the mail-article arrives at the point of the omitted mail detector. With increasing omitted mail detectors, this timing error is extended. Consequently, a mis-tracing operation occurs when the timing error exceeds the transport time corresponding to the distance of one section, i.e., the distance between two adjacent mail detectors in the light bar method. Since the number of mail detectors which can be omitted is limited to several in practice, it is not possible to reduce the system hardware, such as mail detectors, remarkably.